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It depends upon your actual OTA receiver and your setup. My two Panny receivers' ATSC tuners will normally lock on a steady OTA signal of at least 30%. My HR20-700's ATSC needs a bit more to lock on. If you have signals steady above 80% you should be fine in most cases.

It depends on what your talk about. +66.66 dBm will light up your house at night. +66.6 dBmv will overdrive you tv big time. +66.6 dbuv is a little low. A SQ of +66.6 is low. but, will give you a constant picture.

Getting down to the nitty gritty, there's no definitive answer, because it depends on the sensitivity of your receiver AND the signal level from your antenna. Without professional measuring equipment, you can't tell the antenna output. Receiver sensitivity depends upon make and model. Signal levels you see from a reading of the "signal meter" on your receiver aren't always reliable.

Those numbers are at the top of the range because you are less than 5 miles from the transmitters (2 of them any way).

These numbers are not too high, they are normnal for your proximity.

They simply preclude you from using amplification to try to get distant stations.

If, for instance, you want to receive the FOX station which is RF channel 23 at 41 miles and 13.7dB, you might be able to use a Winegard HDP-269 pre-amp but CBS channel 13.1 (RF 38) is in the same direction but 12 miles out may be a problem.

One way to help might be to use a jointenna as a narrow band attenuator (one leg of a channel 38 jointenna passes channels 36-40 and the other blocks it). If you can set up 2 jointennas for ch 8 and ch 11 to trap out 8 and 11 (your biggest problems) you might be able to rig a setup that will let you use a more powerful amplifier, otherwise forget it and move to a location more than 15 miles from a transmitter.

Any value in dB is not an absolute power measurement, it is a relative measurement used to compare two values. For example, on the tvfool chart, NM provides the noise margin. This is how high (or low) the signal is compared to the noise. After adding and subtracting the gain, losses, and noise of your receiving system you want to wind up with a positive number.

When talking about pre-amp overload the value to look at is the power in dBm. This is an absolute measure of power. 0 dBm is equal to 1 mW. 3 dBm equals 2 mW. 60 dBm = 1000 watts. 80 dBm = 100,000 watts.

To determine if your pre-amp will overload you must add all of the signals it will receive and amplify: VHF, FM, UHF, plus any nearby transmitter in the amplifier's pass band including non-broadcast. I tried to find the overload spec for a CM7777 pre-amp, but so far no luck. I did find on one dealer's web site where it said this pre-amp could handle 350,000 uV. That's 1.7 mW into 75 ohms, or about +2 dBm. I have no idea if that spec is accurate, but it gives you and idea. Again, this is for the total power of all signals amplified. The gain of the antenna before the pre-amp must also be considered.